Generally, a VLF TD measuring method in a power system is the most representative method for measuring a TD change in cables or power facilities with various kinds of applied voltages to diagnose abnormal signs such as the occurrence of water trees or gaps in an insulator.
When a water tree occurs inside an insulator that has run for a long time, a degradation phenomenon occurs, which includes a decrease in insulation resistance and an increase in loss current. This phenomenon is represented as a TD change, and whether an abnormality is present and the state of degradation are determined by measuring the amount of this change. In particular, since a high voltage insulator has very high insulation resistance and capacitance, a leakage current and voltage exhibit a phase difference of 90°. However, this is a theoretical conclusion appearing in an ideal state circuit. In reality, a slight deviation occurs due to a resistance component inside the insulator, and at this point, the phase angle deviation of voltage-current is represented as TD. In other words, a large numerical TD value indicates the occurrence of an abnormality.
As the foregoing method of measuring TD, there is a measurement method according to a Schering bridge principle and a measurement method using the minute phase difference between voltage and current as disclosed in Japanese Patent Application Laid-Open Publication No. 1996-201455. At this point, the TD measuring method using the phase difference normally employs a VLF signal of about 0.1 Hz. The reason is that whether the water tree is present may not be easily diagnosed with a TD value change at practical commercial frequency of 60 Hz, but may be diagnosed only with a TD value change at a VLF of 0.1 Hz or lower.
However, since the method for measuring TD using VLF determines a state of degradation based on a simple comparison between a certain reference value or level value and a measured value, there are limitations in that since there is no statistical background, an individual country's environmental conditions, which include a ratio of underground cable installment, rainfall, flood characteristics, management state, cable type, and development characteristics, are not considered.
In addition, the method for measuring TD using VLF does not suggest clear determination criteria, such as numerical reproducibility, and lacks standardization for application to the field. FIG. 1 shows new determination criteria of VLF TD proposed by IEEE in 2010, which are greater determination values compared to the previous ones. At this point, since, as measurement factors depending on the cable type, a standard deviation for a measurement condition of 1 Uo is added by a logical OR operation, a state determination range is set wide, from 4 to 50, which causes much confusion.
In addition, since the method for measuring TD using VLF relies on techniques from advanced European countries, there are limitations in that since accurate determination of the cause of occurrence of VLF TD is not possible, it is difficult to actively respond to facility failure or facility malfunctions, such as insulation breakdown, and accordingly facility reliability is lower under various field conditions.